The present invention relates to an exhaust emission control device for cleaning exhaust gas discharged from an internal combustion engine capable of lean burn.
Examples of an exhaust emission control device for cleaning exhaust gas discharged from an internal combustion engine capable of lean burn, such as a diesel engine or a lean burn gasoline engine, include NOx catalysts, such as a selective reduction type NOx catalyst and an occlusion reduction type NOx catalyst.
The selective reduction type NOx catalyst is a catalyst which reduces or decomposes NOx in an atmosphere with excess oxygen in the presence of hydrocarbon (HC). To clean NOx with the selective reduction type NOx catalyst, an appropriate amount of HC component (hereinafter referred to as reducing agent) is required.
When using the selective reduction type NOx catalyst for the exhaust emission control of an internal combustion engine as mentioned above, the amount of HC component in the exhaust gas during normal operation of the internal combustion engine is very small, so that in order to clean NOx during normal operation, it is necessary to supply as a reducing agent, for example, light oil serving as fuel to the selective reduction type NOx catalyst.
On the other hand, the occlusion reduction type NOx catalyst absorbs NOx when the air fuel ratio of the inflow exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration of the inflow exhaust gas is reduced, effecting reduction to N2.
When using the occlusion reduction type NOx catalyst for exhaust emission control of the internal combustion engine, the NOx in the exhaust gas will be absorbed by the NOx catalyst since the air fuel ratio of the exhaust gas during normal operation of the internal combustion engine is lean. However, when exhaust gas of lean air fuel ratio is continued to be supplied to the NOx catalyst, the NOx absorbing capacity of the NOx catalyst attains saturation, and no further NOx can be absorbed, with the result that the NOx in the exhaust gas is allowed to leak.
In view of the above, in the occlusion reduction type NOx catalyst, it is necessary to reduce the oxygen concentration by making the air fuel ratio of the inflow exhaust gas rich with a predetermined timing before the NOx absorbing capacity attains saturation, and to release the NOx absorbed by the NOx catalyst to reduce it to N2, thereby recovering the NOx absorbing capacity of the NOx catalyst. In the following, this operation of temporarily making the air fuel ratio of the inflow exhaust gas rich will be referred to as rich spike.
On the other hand, to recover the NOx absorbing capacity of the NOx catalyst, it is necessary to appropriately enrich the air fuel ratio of exhaust. Conventionally, the pattern in which fuel as reducing agent is added and the target fuel addition pressure are set on the basis of a map indicating the relationship between the internal combustion engine RPM and fuel injection amount obtained through experiment.
However, when supplying reducing agent at the exhaust port of an internal combustion engine, the exhaust port and the NOx catalyst are usually spaced apart from each other, and the reducing agent is not easily carried by the exhaust flow depending upon the operating condition of the internal combustion engine, resulting in a rather poor efficiency in the supply of reducing agent. Specifically, in the range where the internal combustion engine operates at low speed and with small load, the exhaust velocity is low, and the exhaust gas temperature is low, so that a part of the reducing agent added adheres to the wall surface in the exhaust passage, with the result that the degree of rich spike at the exhaust port differs from the degree of rich spike at the NOx catalyst. In other words, even if the rich spike on the exhaust port side attains the target air fuel ratio, the amount of reducing agent reaching the NOx catalyst decreases due to the adhesion to the wall surface on the NOx catalyst side, resulting in a decrease in the degree of richness. Further, the rich spike, effected instantaneously, involves a delay in reaction on the NOx catalyst side, with the result that the rich period is extended and the degree of richness becomes lower than the target air fuel ratio.
For example, even if the degree of rich spike at the exhaust port is approximately the one as shown in FIG. 6(A), the degree of richness is lowered with the NOx catalyst as shown in FIG. 6(B). Thus, the target air fuel ratio cannot be attained with the NOx catalyst, and the release and reduction of NOx cannot be effected to a sufficient degree. Then, in some cases, the NOx absorbing capacity of the NOx catalyst attains saturation and cannot be recovered, resulting in leakage of NOx in the exhaust.
The present invention has been made in view of the above problems. It is an object of the present invention to provide an exhaust emission control device for an internal combustion engine capable of supplying an appropriate amount of reducing agent to NOx catalyst even if the operating condition of the internal combustion engine changes.
The present invention relates to an exhaust emission control device for an internal combustion engine, characterized by comprising:
an NOx catalyst provided in an exhaust passage of an internal combustion engine capable of lean burn and adapted to release absorbed NOx by a reducing agent for reduction;
a reducing agent supplying means provided in the exhaust passage on the upstream side of the NOx catalyst;
a load detecting means for detecting the load of the internal combustion engine; and
a reducing agent addition control means for controlling reducing agent addition period and reducing agent addition interval on the basis of the load of the internal combustion engine.
In accordance with the present invention, it is desirable to further provide an addition determining means for determining as to whether reducing agent is to be added or not according to the operating condition of the vehicle, the addition amount of reducing agent and the reducing agent addition interval being controlled on the basis of the load detected by the load detecting means when it is determined by the addition determining means that addition is possible.
It is desirable that a judgment be made by the addition determining means as to whether the NOx catalyst is at an activation temperature or not, whether the operation range of the internal combustion engine is within the range where addition of reducing agent is possible or not, and the like, reducing agent being supplied only when release and reduction of NOx is possible, thereby preventing the reducing agent from passing through the NOx catalyst.
Further, when the load of the internal combustion engine is small, it is possible to increase the amount of reducing agent added and elongate the reducing agent addition interval as compared to the case in which the load of the internal combustion engine is large.
Further, the present invention relates to an exhaust emission control device for an internal combustion engine, characterized by comprising:
an NOx catalyst provided in an exhaust passage of an internal combustion engine capable of lean burn and adapted to release absorbed NOx by a reducing agent for reduction;
a reducing agent supplying means provided in the exhaust passage on the upstream side of the NOx catalyst;
an operating condition detecting means for detecting the load and an RPM of the internal combustion engine; and
a reducing agent addition control means for controlling reducing agent addition period and reducing agent addition interval on the basis of the detected load and the RPM of the internal combustion engine.
In the present invention, when the load and the RPM of the internal combustion engine are low, the velocity of the exhaust is low, and the reducing agent is not easily carried by the exhaust flow, so that the addition period and addition interval for the reducing agent added are controlled according to the engine load and the engine RPM. That is, when the load and RPM of the internal combustion engine are low, the addition period and addition interval for the reducing agent are increased. On the other hand, as the engine load increases, the addition period and addition interval for the reducing agent are reduced. The addition period and addition amount for the fuel are in a fixed relationship unless the addition pressure (fuel pressure) of the reducing agent varies, so that the addition amount of the reducing agent is in proportion to the addition period thereof.
Thus, in accordance with the present invention, the addition period and the addition interval for the reducing agent are controlled while taking into account, for example, the degree to which reducing agent adheres to the wall surface in the exhaust passage depending on the velocity, temperature, etc. of the exhaust of the internal combustion engine, making it possible to always supply a sufficient amount of reducing agent to the NOx catalyst side independently of the operating condition.
Further, when the load and RPM of the internal combustion engine are low, the amount of NOx generated is small, so that, by elongating the addition interval, the reducing agent previously added interferes with that subsequently added in the condition in which the velocity of the exhaust is low, whereby it is possible to prevent the degree of richness from increasing excessively.
Regarding the exhaust emission control device of the present invention, examples of an internal combustion engine capable of lean burn may include an in-cylinder direct injection type lean burn gasoline engine and a diesel engine.
The detection of the load by the operating condition detecting means can be effected on the basis, for example, of an output signal of an accelerator opening sensor or an output signal of an airflow meter indicating intake air amount. The detection of the engine RPM can be effected through calculation of an output pulse, for example, of a crank angle sensor.
Examples of the NOx catalyst in the exhaust emission control device of the present invention may include an occlusion reduction type NOx catalyst and a selective reduction type NOx catalyst.
The occlusion reduction type NOx catalyst is a catalyst which absorbs NOx when the air fuel ratio of inflow exhaust gas is lean and releases the absorbed NOx for reduction to N2 when the oxygen concentration of the inflow exhaust gas decreases. The occlusion reduction type NOx catalyst uses, for example, alumina, as the support, which supports thereon, for example, at least one selected from the group consisting of: an alkali metal, such as potassium K, sodium Na, lithium Li, or cesium Cs, an alkaline earth metal, such as barium Ba, or calcium Ca, and a rare earth metal, such as lanthanum La or yttrium Y, and a precious metal, such as platinum Pt.
The selective reduction type NOx catalyst is a catalyst which reduces or decomposes NOx in an excess oxygen atmosphere and in the presence of hydrocarbon and it includes a catalyst in which zeolite supports a transition metal, such as Cu, that has undergone ion exchange, a catalyst in which zeolite or alumina supports a precious metal, and the like.
In the exhaust emission control device of the present invention, the reducing agent adding means may be formed by a reducing agent supply pump, a reducing agent injection nozzle provided in the exhaust passage, etc.
In the exhaust emission control device for an internal combustion engine of the present invention, even if the reducing agent adding position and the NOx catalyst are spaced apart from each other, the reducing agent can be supplied in an appropriate manner by controlling the addition period and the addition interval for the reducing agent. In particular, when the RPM and load of the internal combustion engine are low, the addition period and addition interval for the reducing agent are elongated, whereby, even if a part of the reducing agent adheres to the wall surface in the exhaust passage, it is possible for a sufficient amount of reducing agent to be carried to the NOx catalyst by the exhaust flow.
In this way, independently of the operating condition of the internal combustion engine, it is possible to cause an appropriate amount of reducing agent to reach the NOx catalyst, thereby effecting NOx reduction efficiently. Thus, it is actually possible to constantly maintain the NOx occlusion amount by the NOx catalyst at a level near zero, thereby achieving a high level of efficiency in NOx cleaning.